1. Field of the Invention
The present invention relates to a dual-phase steel sheet of excellent fatigue characteristic in a spot welded joint. More in particular, it relates to a superhigh-strength dual-phase steel sheet having a tensile strength of about 780 to 1270 MPa.
2. Description of the Prior Art
Recently, demands for improvement of safety on automobiles have been increased more and more. From the view points of ensuring the drivers"" safety in a car crash as well as improving the fuel cost by reducing the weight of car bodies that has been increasing owing to attachment of safety equipments, the technique of applying high strength steel sheets to frame portions of car bodies has come to be adopted rapidly. Especially for preventing the frame portions from being flexed and intruded into a cabin at the time of the side impact, there comes to be used superhigh-strength steel sheets having a tensile strength of about 780 to 1180 MPa.
The superhigh-strength steel sheets for car components have generally employed dual-phase steel sheets comprising ferrite and martensite with the ferritic microstructure being as a matrix phase in which: coarse island martensite is dispersed at the triple point of the ferrite grain boundary; or martensite is connected in a network-shape. Nevertheless it is generally considered that it is difficult to ensure sufficient ductility in superhigh-strength steel sheets of 780 MPa or more, the dual-phase steel sheets have been employed. This is because the jig sheets can improve the ductility by the soft ferrite microstructure and also ensure a predetermined strength by the martensitic microstructure. This permits of steel sheets excellent both in the strength and the ductility and also excellent in the weldability.
The dual-phase steel sheets are disclosed in JP-A Nos. (1) 128320/1992, (2) 173946/1992, and (3) 105960/1993. Each of them has superhigh-strength of 780 MPa or more and excellent ductility. However, the gist of these techniques is to make steel sheets compatible with strength and formability. Thus, when the tensile strength in the superhigh-strength steel sheet increases to about 780 to 1180 MPa as shown in the present invention, the amount of elements such as C, Mn that ensure strength tends also to increase remarkably even on a dual-phase steel sheet, causing the lowering of weldability. Currently, no effective means for the defect has not yet been studied.
Generally, dual-phase steel sheets have two problem: since spot welded nugget portions (lens-shaped molten and solidified portion formed when metal sheets are stacked to each other and spot welded) tend to be hardened while the heat-affected zone (HAZ) is tend to be softened, difference of hardness between them increases; and defects such as micro-cracks are formed near the weld zone including the welded nugget portions. These cause the fatigue characteristic to be lowered remarkably, particularly, on the welded joint portion. The steel sheets described above also involve the same problems in the conventional dual-phase steel sheets and improvement has been demanded keenly for the fatigue characteristic of the spot welded joint.
On the other hand, examples for improving the strength of the welded joint portion are described in JPA-Nos. (4) 199343/1991, (5) 186849/1993, and (6) 87175/2000.
Of these, (4) is directed to extra-low carbon steels with C content of 0.006% or less. Thus no desired superhigh-strength can be obtained; (5) and (6) are intended to prevent the heat-affected zone from softening like in this invention. However, since predestined plastic strain is applied to a steel sheet for work hardening, the ductility is lowered remarkably, so these are not practical.
Accordingly, strongly demanded is a novel dual-phase steel sheet having high strength and ductility that is improved with the fatigue characteristic in the welded joint portion.
Under the circumstances, the present invention aims at providing a superhigh-strength dual-phase steel sheet having strength of about 780 to 1180 MPa, as well as being improved in the fatigue characteristic for the welded joint portion.
In carrying out our invention in one preferred mode, we utilizes superhigh-strength dual-phase steel sheet that is a ferritic microstructure and martensitic microstructurexe2x80x94containing dual-phase steel sheet containing:
C: 0.08-0.20% (mass% here and hereinafter),
Si: 0.5% or less (inclusive of 0%)
Mn: 3.0% or less (exclusive of 0%)
P: 0.02% or less (inclusive of 0%)
S: 0.02% or less (inclusive of 0%), and
Al: 0.001-0.15%, further containings
Mo: 0.05-1.5%, and
Cr: 0.05-1.5%, and satisfying:
the average Vickers hardness of the ferritic microstructure of 150 Hv or more and the average Vickers hardness of the martensitic microstructure of 500 Hv or more.
Other and further objects, features and advantages of the invention will appear more fully from the following description.